Austenitic stainless steel resistant to stress corrosion cracking

ABSTRACT

AN AUSTENITIC STAINLESS STEEL OF THE (5 NICKEL TYPE WHICH OFFERS SATISFACTORY RESISTANCE TO STRESS-CORROSION CRACKING CONTAINS CORRELATED AMOUNTS OF CHROMIUM, MANGANESE, SILICON, NICKEL, CARBON, NITROGEN, COPPER, ETC.

United States Patent 3,806,337 AUSTENITIC STAINLESS STEEL RESISTANT T0 STRESS CORROSION CRACKING Robin M. F. Jones, Sntfern, N.Y., assignor to The International Nickel Company, Inc., New York, N.Y. No Drawing. Filed Jan. 3, 1972, Ser. No. 215,164

Int. Cl. C22c 39/20, 39/54 US. Cl. 75-125 2 Claims ABSTRACT OF THE DISCLOSURE An austenitic stainless steel of the 8% nickel type which offers satisfactory resistance to stress-corrosion cracking contains correlated amounts of chromium, manganese, silicon, nickel, carbon, nitrogen, copper, etc.

As is generally known, stainless steels have found extensive use in a considerable number of diverse industrial and commercial applications. And this has been conspicuously true in respect of the conventional austenitic steels, probably by reason of the fact that they offer an exceptional combination of corrosion resistance, mechanical characteristics, ease of fabricability, etc. Nonetheless, and such attributes notwithstanding, it also has been these steels which have manifested a distinct propensity to undergo premature stress-corrosion cracking, the 8% nickel type being particularly susceptible.

Despite the numerous theories advanced in explanation of the phenomenon, there is seemingly no single well accepted theory. There is, however, general agreement to the effect that when conventional stainless steels of, say, the widely used AISI 304 type, are subjected to stress, whether it be externally applied or otherwise induced, e.g., as by some processing operation, and the steel is exposed to certain corrosive media, the resultant environment is quite conductive for stress-corrosion cracks to initiate and propagate. Various chlorides have displayed a particularly destructive talent in this regard, the chlorides of magnesium and sodium being illustrative.

As might be expected, many solutions have been proposed to the onset of, if not completely obviate, stress-corrosion failure. One panacea has been to use alloys containing upwards of 40% nickel. Such alloys are excellent but expensive. High cost is also a drawback to the proposals set forth in, for example, U.S. Pats. Nos. 3,159,479, 3,159,480 and 3,523,788. Too, very close control of various constituents are required. Various thermomechanical treatments have been advocated, but an inherent disadvantage attendant such treatments is that base materials often cannot be welded or heat-treated without detracting from other desired properties. To achieve an answer accompanied by an unacceptable sacrifice of some other desired property is not too attractive. Other proposals have included the utilization of special processing techniques, e.g., vacuum melting procedures (often coupled with the use of high purity ingredients).

The foregoing is deemed sufficient to demonstrate that it would be of considerable benefit to have available austenitic stainless steels of the 8% nickel type (1) which afford an improved order of magnitude in terms of resistance to stress-corrosion attack and yet would not (2) necessitate the use of substantial amounts of expensive ingredients, or (3) require recourse to special processing 3,806,337 Patented Apr. 23, 1974 techniques, or (4) be attained at the detriment of other desirable characteristics, etc. It is this overall objective to which this present invention is primarily (through not exclusively) addressed.

It has now been discovered that the susceptibility of austenitic stainless steels containing conventional amounts of nickel, 8% to 10%, to stress-corrosion cracking is significantly reduced provided the steels contain correlated percentages of nickel, chromium, manganese, silicon, carbon, etc., as more fully described herein. Special processing is not required nor is the utilization of the purest materials. Mechanical properties are not deleteriously affected and the cost is hardly above that which prevails for a steel such as A181 304.

Generally speaking, the present invention contemplates austenitic stainless steels containing, by weight percent, at least about 12%, e.g., 13.5% to 24%, chromium; about 1.75%, e.g., 2% to 19%, manganese, the chromium and manganese being correlated such that when the chromium is from 18% to 24% the manganese does not exceed 10%; about 2.75% to about 3.5% or 4%, silicon; about 7% to less than 12% nickel; nitrogen up to about 0.25%; up to about 0.1% carbon; up to 3%, and advantageously, about 0.5% to 1.5%, copper; up to 1% titanium; and the balance essentially iron.

In carrying the invention into practice the chromium content should be at least about 12%; otherwise, corrosion resistance suffers. Care must be exercised with regard to the upper limit lest a detrimental level of sigma and/or delta ferrite be introduced into the austenitic matrix. Delta ferrite and signa (an intermetallic precipitate) are notable for bringing about embrittlement and/ or hot working difiiculties. While the steels should be free of such phases, up to 2% or 5% thereof can be tolerated in hot workable steels. To that end and particularly for hot workable steels, it is beneficial that the chromium not exceed about 19%, a range of 14% to 16% or 17% being satisfactory. For steels in the cast condition the chromium can be extended to 25% or 30% since hot workability is not of concern and this is the characteristic most aflFected by delta ferrite. In this instance a higher level of delta ferrite might be tolerated, e.g., up to 10%.

Silicon is a potent ferrite and sigma former and preferably should not exceed about 3.5%. While the lower percentage might extend down to 2.5 particularly where service conditions are not overly severe, it is to considerable advantage that the percentage of silicon be at least 2.75% in seeking optimum stress corrosion resistance, a span of 2.75% to 3.25% giving excellent results.

With regard to manganese, it contributes to achieving and maintaining the desired austenitic face-centered-cubic structure and is also beneficial for deoxidation and other purposes. [It is at the same time, however, a constituent which, if to the excess, has been found to promote the sigma phase and for this reason it should be controlled so as not to exceed 19%. As noted above, it should not exceed 10% when the chromium is above about 18%.Although it may be as low as 1.5% or even 1%, it is preferred that it be at least 2%, a range of from 2% or 3% to 8% or 9% being satisfactory.

Nickel in addition to promoting the formation of an austenitic structure also counteracts the sigma forming tendencies of chromium, silicon and manganese. For such reasons and for good mechanical characteristics and fabricability, at least 7% or 8% should be present, it being unnecessary that the upper level exceed 11% or 11.5%. The important point is high levels, e.g., 15% or more, are neither necessary nor indispensable in minimizing stresscorrosion attack.

Carbon is also an austenite former and in combination with nickel tends to thwart the subversive roles of sigma and delta ferrite. While as indicated above herein, carbon should not exceed about 0.1%, it can be as high as 0.25% though in such instances an undue amount of carbides may form which will adversely affect other types of corrosion resistance, e.g., intergranular corrosion, in some corrosive media. Actually therefore, it is to advantage that carbon be held to levels below about 0.05% or 0.03%. Titanium is useful to tie up carbon. In this way intergranular corrosion, particularly in heat affected zones of welds, is inhibited.

With respect to copper, it is eflfecti ve in respect of corrosion resistance generally and also in offsetting the territe forming tendency of silicon in particular. Copper in percentages of about 0.5 to 1.5% or 2% is quite beneficial.

A particular advantage of the instant invention is that relatively high nitrogen content can be utilized. This has not been the case historically. Even with high nickel austenitic stainless steels (e.g., 15 nickel, nickel being a known crack inhibitor at high levels), the art has been admonished that nitrogen not exceed about 0.05 For 8% nickel steels, lower levels have been imposed. This introduces recourse to high purity materials and/or special processing techniques and, consequently, higher cost. In accordance herewith, up to 0.25% can be used though lesser amounts, up to 0.15%, for example, may be desired. Nitrogen does stabilize austenite and this is of benefit with the higher chromium, e.g., 18%24% chromiurn, and lower manganese, e.g., less than 5% to 6%,

steels. A content of 0.05 to 0.2% or 0.25 can be used in such steels. However, the significant aspect is that low cost air melting practice can be used with the realization that amounts of nitrogen common to such practice, say 0.04-0.6%, will not be injurious.

For the purpose of giving those skilled in the art a better appreciation of the invention the following data are given.

A series of stainless steels, Table I, were prepared using Armco iron, electrolytic manganese, nickel and copper, metallic silicon, silicon-manganese, vacuum grade chromium, titanium sponge, if any, and calcium-silicon. Nickel and iron were [first charged and heated to approximately 2850 F. after which silicon-manganese and wash metal were added. The heats were killed with calciumsilicon whereupon the chromium, manganese, silicon and copper were added. Aluminum was used for deoxidation. Thirty pound ingots were produced using an air induction furnace and magnesia crucible, pouring being carried out at about 2650 F. Thereafter, the ingots were soaked for 3 hours at 2000 F. and rolled to /2" plate, a portion of which was further rolled to 4" plate. Prior to test the 4" plates were heat treated at 2000 F., air cooled and thereafter machined to /3 specimens.

The specimens were stressed in the form of U-bends and immersed in boiling magnesium chloride (42% concentration, 154 C.). The U-bends were examined after the first, second and 24 hour periods and then put back into the same solution. They were then examined daily for approximately three days. Thereafter, they were examined approximately every seven days, a fresh magnesium chloride solution being used during each subsequent 7 day period. The full test period covered approximately thirty (30) days after which the test was discontinued. During the overall period, specimens which showed evidence of cracking failure were removed from test. Table I includes data obtained with respect to the point in time at which cracking occurred as well as maximum depth of crack penetration. Data for 304 purchased from a commercial source is also given for purposes of comparison.

TABLE I.-STEEL COMPOSITIONS Cracking behavior Percent 01- Time, Depth, Steel N0 Cr Ni Si Mn 0 N Cu days mils 1---- 15.4 7.9 2.79 4.8 0.068 0.02 1.04 30 1.5 2 15.1 8.4 3.23 17.9 0.210 0.016 1.10 30 20 A 14.5 8.3 3.07 23.8 0.064 0.027 1.05 1 120 15.3 8.1 2.74 5.0 0.065 0.025 1.09 30 7 17.9 8.1 2.95 4.7 0.061 0.019 1.03 30 2 15.2 8.2 2.92 4.9 0.023 0.034 1.05 30 None 18.2 9.3 3.05 2.0 0.034 0.14 1.01 30 None 18.0 9.2 3.10 4.75 0.043 0.10 1.0 30 None 8 22.4 9.3 3.04 2.01 0.039 0.23 1.0 30 None .4181 304. Commerieal source 1 1 Hour.

NOTE.All0y 1 contained 0.2% Ti, none added to others. Alloy 2 contained 0.05% Mo, all others less than 0.03%, if any. All steels contained not more than 0.015% P and 0.015% S. (A181 304 not analyzed).

The data in Table *I are indicative of the superiority of steels within the invention in contrast to the widely used, conventional 8% nickel AISI 304. Steel A, a steel outside the invention, is illustrative of the poor results that might be expected with excessive manganese. It should be mentioned that high chromium (22.4%) Steel N0. 9' showed some edge cracking. Though this steel was workable, lowering the chromium to 20% or slightly increasing the carbon content would be helpful.

Tests were also conducted in respect of various steels in accordance herewith using sodium chloride. In this regard, specimens prepared as generally described above were exposed in an autoclave to both the aqueous and vapor phases containing chloride ions, the temperature being maintained at about 500 F. AISI 304 was also exposed for a comparison base. All steels within the invention survived for a period at least twice that of AISI 304.

Mechanical properties are reported in Table II for Steels 1 and 2 /2" plate), in both the As-Rolled and As-Rolled and Annealed condition (1 hr. at 2000 F. folof steels within the invention are comparable to standard 8% nickel austenitic stainless steels. In addition to the above, drawability and stretchability tests on steels within the invention indicate that these properties are also comparable to the 8% nickel stainless steels, drawability being generally slightly better than for A151 304 with stretchability being generally slightly lower.

An advantageous alloy range is as follows: 14% to 17% chromium, about 1.9% to 5% manganese, about 2.75% to 3.5% silicon, about 7.5% to 10.5% nickel, up to about 0.03% or 0.05% carbon, about 0.5% to 1.5% copper, nitrogen in an amount, e.g., 0.02%, up to 0.15%, and the balance essentially iron, are especially suitable in the chemical processing industry where stress-corrosion cracking has heretofore been so prominent. It is to be further pointed out that no impairment in mechanical properties of the austenitic steels within the invention is encountered, the mechanical properties being quite comparable to properties of typical AISI grades of austenitic chromium-nickel stainless steels.

As will be understood by those skilled in the art, the term balance or balance essentially when used in referring to the iron content does not exclude the presence of other elements commonly present as incidental ingredients, e.g., deoxidizing and cleansing constituents, and impurities ordinarily associated therewith in amounts that do not adversely affect the characteristics of the steels. Phosphorus and sulfur should preferably not exceed 0.02% and 0.015%, respectively, although up to 0.2% phosphorus and 0.03% sulfur might be tolerated. Aluminum can be used for oxidation purposes, an amount above 0.2% not being necessary. Molybdenum should be avoided. Carbide formers, e.g., columbium or vanadium can be used in lieu of or together with titanium. Conventional processing practices can be used. Pouring temperatures of 2650 F. to 2850 F. are recommended. Vacuum processing can be employed, though not necessary.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

I claim:

1. An austenitic stainless steel which notwithstanding that it is of the 8% nickel type is characterized by good resistance to stress corrosion cracking when subjected to stress and in contact with halide solutions, said steel consisting essentially of at least about 12% and up to 30% chromium, the chromium not exceeding 24% in the wrought condition, about 1.75% to 19% manganese, the chromium and manganese being correlated such that when the chromium is from 18% to 24% the manganese does not exceed 10%, about 2.75% to 4% silicon, about 7% to less than 12% nickel, up to about 0.25% nitrogen, up to about 0.1% carbon, 0.5% to 3% copper, up to 1% titanium and the balance being essentially iron.

2. An austenitic stainless steel in accordance with claim 1 containing 1.9% to 5% manganese, 2.75% to 3.5% silicon, 7.5% to 10.5% nickel, up to 0.05% carbon, 0.5% to 2% copper, nitrogen up to 0.25%, up to 1% of a carbide former from the group consisting of titanium, columbium and vanadium and the balance iron.

References Cited UNITED STATES PATENTS 3,282,686 11/1966 Allen 128 A 3,523,788 8/1970 Bates 75l28 C 3,615,365 10/1971 McCunn 75-128 A 2,747,989 5/1956 Kirkby 75-128 A 3,615,368 10/1971 Baumel 75-l28 A HYLAND BIZOT, Primary Examiner US. Cl. X.R. 75128 A, 128 C 

